Export Coefficient Table

The export coefficient table lists loading rates for each pollutant type by land use type. The first record (row) of the table must identify the field names starting with land use type followed by the pollutants under evaluation. The table may contain any number of land use and pollutant types. The land use type names must be the same in the table as they are in the GIS land use file. There should be loading rates for each land use and pollutant type in the evaluation area, otherwise the load for the area will be zero. The rates in the export coefficient table are measured in pounds per acre and are typically used to calculate the pollutant loads for rural land use types.
The export coefficient table is developed by water resource engineers generally based on values available from the literature or they can be developed based on analysis of watershed stormwater monitoring data.

Event Mean Concentration

The event mean concentration (EMC) table is identical to the export coefficient table, except that the EMC values are measured in milligrams per liter and typically used to calculate the pollutant loads for urban land use types.
The EMC table is developed by water resource engineers based on values available from the literature or can be developed based on analysis of watershed storm water-monitoring data.

EMC Table

Impervious Factor

The impervious factor table identifies the percentage of imperviousness for each land use type. It is used to calculate the EMC runoff coefficient. The first record (row) of the table identifies the field names comprised of the land use type and percent age of impervious field names. The table may contain any number of land use types, but there should be impervious percentages for each land use type in the evaluation area. If there is no impervious factor in the table for a particular land use type, then the EMC runoff coefficient will default to .05 for areas with that land use. The names describing the land use types must be the same in the table as they are in the GIS land use file.
The impervious factor table is developed by water resource engineers and GIS analysts, by analyzing the impervious surfaces of different land uses on aerial photographs, or by use of literature values.

Impervious Factor Table

Best Management Practices

The BMP table contains percentage removal efficiency multipliers for each BMP type that are used to calculate pollutant load reductions. The first record (row) of the table identifies the field names starting with BMP type followed by the pollutants under evaluation. The table may contain any number of BMP types. The pollutant types without percentage efficiency multipliers will not reduce the pollutant load for the BMP type.
The BMP table is developed by water resource engineers by use of literature values, or by analyzing local monitoring data comparing pollutant loads entering and leaving BMPs.

BMP Table

Computations

The following formulas were used from a study done by TVA in the Buttahatchee Watershed:

Sediment Delivery Ratio

DR = 0.417762*(A^-0.134958) - 0.127097

Where:

DR = sediment delivery ratio
A = area (sq. miles)

Soil Loss

A = R*K*LS*C*P

Where:

A = soil loss (tons/acre/year)
R = rainfall energy factor
K = soil erodibility factor
LS = slope-length factor
C = cropping management factor
P = erosion control practice factor

Pollutant Load

M = A*Area*DR*PC*0.0005

Where:

M = pollutant loading (tons/year)
A = soil loss (tons/acre/year)
Area = land class area (acre)
DR = sediment delivery ratio
PC = pollutant coefficient (pound pollutant/ton soil)
0.0005 = unit conversion factor